1. Field of the invention
The present invention relates to a catalyst for fluorination of 1,1,1-trifluoro-2,2-dichloroethane and more particularly, to a catalyst for producing pentafluoroethane from 1,1,1-trifluoro-2,2-dichloroethane, significantly improved in durability, selectivity and activity and a method for the preparation of the catalyst.
2. Description of the Prior Art
R-502 (a mixture of CFC-115 (CF.sub.3 CF.sub.2 Cl) and CFC-22 (CHF.sub.2 Cl)) has been one of the most important cooling agents utilized extensively in refrigerators, automobile cooling systems, and various other related industries because it is harmless to! the human body and superior in thermodynamic physical properties.
However, intensive research and observation has revealed that CFC-115 is a main substance that destroys the ozone layer in the stratosphere. According to the Montreal protocol internationally agreed in 1987, it is prescribed that CFC-115 should be prohibited from production and use starting from 1996.
Pentafluoroethane (CF.sub.3 CHF.sub.2 : hereinafter referred to as "HCFC-125"), one of important substitutients for R-502 has similar physical properties to R-502 when it is used with HFC-32 (CH.sub.2 CF.sub.2), HFC-143a (CF.sub.3 CH.sub.3) or HFC-134a (CF.sub.3 CH.sub.2 F) as well as does little damages to the ozone layer and has a much less influence to the earth's greenhouse effect.
HCFC-125 may be produced by reacting various C.sub.2 compounds with HF, for example, by reacting HCFC-123 (CF.sub.3 CHCl.sub.2) with HF as shown in the following reaction formula: EQU 2 CF.sub.3 CHCl.sub.2 +3 HF.fwdarw.CF.sub.3 CHClF+CF.sub.3 CHF.sub.2 +HCl
In the above reaction, though 1,1,1,2-tetrafluoro-2-chloroethane (CF.sub.3 CHClF: hereinafter referred to as "HCFC-124") is also produced, it is removed and can be fluorintated to HCFC-125.
However, it is known that the substitution of fluorine for chlorine in HCFC-123 is a very difficult reaction. Thus, an effective catalyst is required to promote this reaction. As previously known, chromium oxide (III) (Cr.sub.2 O.sub.3)-containing catalyst is effective for the partial fluorination reaction. In many a patent, techniques for the preparation of a chromium oxide-containing catalyst are disclosed.
Chromium oxide (III) catalysts developed thus far, however, include many disadvantages that need to be improved, such as catalyst life span and selectivity. For example, since the partial fluorination reaction proceeds at high temperatures, e.g. above 350.degree. C., organic compounds, a reactant for the partial fluorination reaction or a product of the reaction, are decomposed to deposit carbons on the catalyst. As a result, the catalyst is deactivated at a rapid rate.
In order to retard the deactivation rate of the catalyst, a process of supplying oxygen together with the reactants has been reported. However, this process does not completely prevent the deactivation of the catalyst. In addition, the supplied oxygen oxidizes the HCl resulted from the partial fluorination reaction to generate a chlorine gas and water. The generated chlorine gas, in turn, reacts with the reactants to yield many by-products for example, CFC-113 (CF.sub.3 ClCFCl.sub.2), CFC-114 (CF.sub.2 ClCF.sub.2 Cl) and CFC-115 (CF.sub.3 CF.sub.2 Cl) which make the separation and filtration of HCFC-125 or HCFC-124 difficult. The water by-produced is acidified by dissolving the coexisting HCl and HF in a reactor. The acid solution causes corrosion of the equipment including the reactor, and thus has a serious effect on the durability of the equipment.
Many methods for preparing HCFC-124 or HCFC-125 from fluorination reaction in the presence of chromium oxide catalyst have been developed. For example, U.S. Pat. No. 3,258,500 discloses a method for fluorinating tetrachloroethylene in the presence of alumina-supported chromium oxide catalyst at the temperature of 400.degree. C. In this method, however, the selectivity of HCFC-123, HCFC-124 and HCFC-125 is at most 3.5%, 9.2% and 35.5%, respectively.
In U.S. Pat. No. 4,843,181, a method for fluorinating tetrachloroethylene using chromium oxide catalyst obtained by pyrolyzing ammonium dichromate at 500.degree. to 650.degree. C. is suggested. In this method, the total selectivity of HCFC-123, HCFC-124 and HCFC-125 is merely in the range of 71.1.about.90.07%.
Japanese Laid-Open publication No. 4-29940 suggests a method for producing HCFC-123, HCFC-124 and HCFC-125 from HCFC-122 (CF.sub.2 ClCHCl.sub.2) in the presence of alumina containing chromium catalyst with supply of oxygen. Although the total selectivity of HCFC-123, HCFC-124 and HCFC-125 is about 99%, the starting material, HCFC-122 is expensive and has difficulty in producing itself.